Well in my opinion it doesn't have to do with the body type you are born with (tall,short,big,small e.t.c.).It has to do more with the state of one's body ragardless of its type.
One needs to be relaxed,and smooth in order to be able to establish contact with the attacker and keep it,so i certainly agree with Carsten in that one.Usually smaller people have an advantage because they cannot rely on strength in the first place and they can learn correct technique more easily,while bigger people have first to "unlearn" using muscular strength in order for the ellements of correct technique(ki,kokyu,tai sabaki e.t.c) to be able to work.
For the second part of the original poster's question, i don't know really.But i have heard that O'Sensei, even though he belonged in Omoto Kyo himself,he used to say that this has no effect in learning the art of aikido...